2014 i3 12V Battery charge

[Mat Kramer]

Hello. I have a 2014 i3 with no range extender. Recently I went to start it after it had been sitting idle for a few weeks and I started getting a slew of errors starting with "drivetrain failure". A little bit scary but after some googling I determined it was a depleted 12V battery. I tested the battery and it was low. I put it on a trickle charger overnight and in the morning it seemed back to normal. While I was charging I ordered an OBD reader and BimmerLink. I saw a bunch of errors, mostly due to undercurrent but all auto-cleared. I have been driving it normally since then.

So my question is, should I get a new 12V battery? I assume it's not normal for the battery to deplete after a few weeks, but I'm not sure if it was slowly degrading or not. I can keep an eye on the voltage now. I'm not sure how old the battery is. I bought the car used almost 2 years ago. I'm thinking this event indicates a weak battery and with winter coming (Minnesota) I should probably get a new one.

Also, it's not clear to me how the 12V battery gets charged. Does it only charge while driving and is it only from the regenerative braking or is there an alternator continually charging? Does it charge at all from the L1 charger connection?

Thanks!

 

[EvanstonI3]

Get a new battery. A healthy battery would have been fine sitting for 2 weeks.
The 12v battery is charged when:
The car is “ON” driving or not does not matter.
When the car is being charged.
When the battery voltage discharges to 12.0 volts, the car will come on by itself to boost the 12v for exactly One Hour ( there is a possibility that this does not happen on some early models though)

There is no “alternator” in this car.
Instead there is a DC-DC converter that takes the High Voltage from the main battery and converts it to 14v to charge the 12v battery.